The broad purpose of this proposal is to study the intrarenal mechanisms controlling sodium reabsorption. In the proposed experiments we plan to utilize our recent observation that a prompt partial reduction in renal artery pressure prevents the natriuresis induced by ECF volume expansion, to attempt to uncover the factors involved in the control of sodium reabsorption by the kidney. The proposed experiments are designed to determine where within the nephron a reduction in renal artery pressure prevents the decrease in sodium reabsorption normally induced by volume expansion. We also plan to examine: 1. Whether the natriuretic response, once established, can be reversed by a partial reduction in renal artery pressure. 2. The role of changes in medullary solute content in determining the natriuretic response to acute ECF volume expansion. 3. The influence of acid-base disorders on the pattern of distal sodium delivery and reabsorption.